The present invention is directed to the field of rubber mold centrifugal spin casting and more particularly to centrifugal spin casters used therefor.
Spin casting technology has been used for many years to produce costume jewelry, ornamental items, and has recently been employed to produce precision, high strength metal castings as well as thermoset plastic moldings, and wax investment casting patterns for engineered products. Spin casting offers the advantages of economical prototype and low volume production, short production lead time and low cost tooling. Further, it produces precision parts with close tolerances, smooth surfaces, and excellent detail at low per piece cost.
Basically, spin casting involves the use of a pair of complementary disc shaped rubber mold halves which are formed with a series of cavities therein. When the complementary rubber mold halves are placed together, a multi-cavity mold is formed. The multi-cavity rubber mold is clamped by air pressure between metal plates in a motor driven table. As the mold spins on the table, molten metal, liquid plastic or molten wax is poured into the center sprue of the mold, and the metal, plastic, or wax, forced outwardly through radial passageways or "runners" in the mold by centrifugal force, fills the mold cavities. Spin speeds typically range from 100 to 1000 RPM, depending on the size of the mold and the material being cast. The rotation may then be stopped, the mold opened, and the cast parts removed.
Heretofore, spin casters have been made such that access to the rotating turnable is from the top of the machine. The spin casters are typically provided with a horizontally disposed turntable having a plurality of vertically disposed clamp retainers. In one type of system for example, a central opening is provided in the turntable through which an hydraulic push rod is disposed. The push rod bears against a mold ram plate on top of and parallel to the turntable. The mold is adapted to be placed on top of the mold ram plate and a heavy metallic mold cover plate is placed on top of the mold. The mold coverplate is arranged such that clamp stays fixed thereto are placed underneath, and thus captive by, the clamp retainers attached to the turntable. After so adjusting the mold coverplate, the push rod may be actuated to thereby push the mold ram plate into firm engagement with the mold. In other systems, such as a "pancake" O-ring air clamping system, the pushrod is replaced by a hollow steel tube which allows the mold ram plate to be moved upwardly by forcing air directly against the bottom of the plate. In either case, the turntable, along with the mold and plates, are then rotated from the bottom of the machine. Molten metal, plastic, or wax material is poured through a central opening in the mold coverplate and thus delivered to the mold. After the spinning cycle is completed, the turntable, mold plates and the mold are brought to a stop, and the mold ram plate is separated from the mold coverplate. The mold coverplate is then released from the clamp retainers and lifted vertically from the machine by the operator to thus expose the mold. The mold is then removed by also lifting it vertically from the machine to complete the process.
There are many drawbacks associated with the above-described use of the prior art spin casting machines. The operator is burdened with several heavy lifting operations, specifically the vertical lifting into and from the machine of the mold and the heavy coverplate. Since two such operations (in the upward and downward directions) are required for both the mold and the coverplate, operation of the spin caster results in operator fatigue. Since the use of a heavy turntable and upstanding clamp retainers is required, the total spinning mass is high, resulting in slow startup and stopping times. This, coupled with the number of steps involved in placing the mold into and removing it from the spin caster result in very inefficient operation. Further, the upstanding clamp retainers present a serious hazard to the operator since speeds of up to 1000 RPM are not uncommon. Since the turntable, plates and the mold are turned from the bottom of the spin caster, the prior art spin casters are quite large in size and require an elaborate mold clamping/drive pulley mechanism for isolating the rotational movement of the apparatus from the hydraulic unit which provides the mold clamping pressure. Finally, since the mold and mold coverplate are hand loaded from the top of the machine, it is extremely difficult to utilize automatic liquid feeding devices.